1. Field of the Invention
The purpose of the present invention is to make nanostructured ceramic nitride powders at low processing temperatures by nitriding molecular precursors in a nitrogen containing environment including ammonia. The resultant nanostructured ceramic nitride powders can be plastically deformed, making them very desirable for thermo-management material, as for example, in electronic applications.
2. Description of the Related Art
Nitrides, such as aluminum nitride, have exceptional electronic and structural properties. Aluminum nitride (AlN) ceramics have exceptional properties for electronic and structural applications. With a high thermal conductivity and electrical resistivity, a permittivity almost the same as alumina and a low thermal expansion, AlN ceramics are attractive materials for electronic substrates.
Most advanced functional and structural ceramic parts are fabricated by consolidation and densification of powders. For this purpose it is desirable that the starting powders have a small particle size (preferably submicron) and a controlled size distribution.
There has therefore been a growing interest in nanoscale ceramic powders (particle diameter &lt;1000 nm and preferably less than 100 nm), and in the possibility of consolidating these nanoscale ceramic particles to full density utilizing lower processing pressures and temperatures than those required using conventional larger diameter powders.
There has also been a growing interest in the possibility of processing bulk ceramics consisting of consolidated nanoscale particles using near net-shape forming techniques, embossing or pressing rather than grinding, cutting or etching, because of the enhanced ductility of small-grain solids.
Currently, ceramic powders with sizes which are several microns or larger cannot be used in near net-shape forming due to the brittle nature of densified bulk material. Near net-shape forming is forming a product to a final shape by using an appropriate die in a pressure-assisted, thermally activated consolidation process. If a ceramic part having a complex geometry can be formed to near net-shape during processing, the conventional methods of costly precision machining can be avoided.
Nanostructured powders have been prepared by physical vapor deposition, mechanical blending and mixing, and by using chemical routes. However, vapor methods are not cost effective and are used to make small amounts of material. The mechanical blending route often introduces impurities into the final product.
Industrial AlN has been produced by the reaction of nitrogen with aluminum or by the carbothermal reduction of alumina. However, in these processes, the precursor powders are typically very large (i.e., -325 mesh, 45 microns or smaller) and the final powders are not suitable for near net-shape forming. Additionally, these processes require the use of temperatures as high as 2000.degree. C. The resulting material needs to be milled in order to obtain the final finer powder. The milling process generates powders with a wide size distribution and impurities from the mill. While the carbothermal process produces a smaller grain sized material than the direct nitridation of Al, it leaves carbonaceous material mixed with the AlN which needs to be removed.
Other approaches for AlN synthesis include vapor deposition and conversion of chemically derived precursors such as metallo-organic or sol-gel derived. By using chemical methods, composites exhibiting molecular and homogeneous mixing may result. Nanocomposite powders of AlN-BN derived by the pyrolysis of a precomposite gel prepared from an aqueous solution containing the constituent salts have also been studied by Chow et al. Tuesel and Russel have shown that coatings of AlN on SiC can be prepared by the pyrolysis of a polymeric polyiminoalane precursor at 900.degree. C. in ammonia.
It is therefore an object of the present invention to provide a process for making nanostructured aluminum nitride powders in bulk quantity.
It is another aspect of the present invention to provide a process for making nanostructured aluminum nitride powders that can be formed into dense AlN substrates by near net-shape forming.
It is another object of the present invention to produce large quantities of material while achieving chemical homogeneity due to the mixing of constituents at the molecular or atomic level.
It is another object of the present invention to provide a process for forming nanostructured aluminum nitride powders utilizing lower temperatures.
It is yet another object of the present invention to provide a process for making aluminum nitride powders utilizing chemical routes which do not require extensive processing equipment, keeping the cost of production low.